The general processing steps in conventional hydroforming will be explained below using FIG. 1.
First, a metal tube 1 shorter in length than the mold is placed inside a groove of the lower mold 2 so that the tube ends of the metal tube 1 are positioned inside from the end faces of the mold (same figure (a)).
The metal tube 1 of this example is an example of a straight tube. In the case of a bent tube, it is necessary to perform the bending in advance so as to become a shape matching the groove of the lower mold 2.
Next, the upper mold 3 is lowered to close the mold and clamp the metal tube 1 between the lower mold 2 and the upper mold 3 (same figure (b)).
After that, the seal punches 4 and 5 are made to advance. Water is inserted as a pressurizing fluid from the seal punch 4 having a water insertion port 6 while making the punches advance. Substantially simultaneously with the water 7 being filled inside the metal tube 1, the seal punches 4 and 5 are made to contact the end faces of the metal tube 1 to seal them to prevent the water 7 from leaking (same figure (c)).
After that, the pressure inside the metal tube 1 (below, referred to as the internal pressure) is raised to obtain the hydroformed product 8 (same figure (d)). To prevent the water 7 from leaking and secure a seal at this step, the cross-sectional shape of the tube ends 9 of the metal tube 1 and the tube end vicinities 9′ may be made the same circular shapes as before being worked.
However, when the end faces of the final product 10 are not the same shapes as the tube material, since the tube ends 9 and tube end vicinities 9′ and the transition parts 11 are unnecessary, they are cut off and discarded (same figure (e)). That is, the yield falls by that amount.
An example reducing this drop in yield is described in “Automobile Technology (vol. 57, no. 6 (2003), p. 23)”. In this example, the tube ends are not circular, but are rectangular in cross-section the same as the end face shapes of the final product shape. However, in this case, before placing the metal tube to the mold, pre-forming for forming the tube ends into rectangular cross-sections becomes necessary.
In the method described in Japanese Patent Publication (A) No. 2004-42077, a metal tube with a circular cross-section is placed as it is to the lower mold so that the tube ends of the metal tube become inside the end faces of the mold. Along with the descent of the upper mold, the tube ends are made to deform to rectangular cross-sections. The rectangular cross-section seal punches are made to abut against these as is, then the pressurizing fluid is supplied to the inside of the metal tube for axial pressing as necessary. However, while this method can be applied to elliptical, rectangular, oblong, and other relatively simple cross-sections, the front ends of the seal punches must be formed to the same shapes as the ends of the shaped article. Application to complicated cross-sections is considered difficult.
Further, to prevent wrinkles forming at the time of closing the hydroforming mold, the practice has been to close the mold while applying internal pressure. With the method, it is necessary to seal the tube ends after finishing closing the mold, so for example as described in Japanese Patent Publication (A) No. 2001-9529, the method is adopted of closing the mold at just the tube ends and pushing the seal punches to secure a seal, then closing the mold at the tube center. Accordingly, the tube ends in this case are limited to a circular, elliptical, or other simple cross-sectional shapes.
On the other hand, hydroforming has the defect of the difficulty of spot welding and bolting with other parts after shaping. Therefore, technology for forming a flange at the time of hydroforming is proposed in Japanese Patent Publication (A) No. 2001-259754 or Japanese Patent Publication (A) No. 2006-61944. However, with these methods, pluralities of hydroforming steps or separate punches able to move in the mold become necessary. Further, with the method, it is believed difficult to form a flange along the entire length while applying internal pressure.